The overarching goal of the proposed studies is to characterize the molecular interactions between Tribbles proteins (TRIBs) and the E3 ligase COP1 in the formation of AML, harnessing insights from the structural studies as the foundation for future discovery of targeted inhibitors of this complex. We also intend to identify additional targets of TRIB-COP1-mediated ubiquitination and degradation relevant to human disease. TRIB overexpression drives leukemia formation in mice with 100% penetrance. Expression array studies of human AML samples support a role for TRIB in human leukemia. In the mouse model, TRIB's ability to inhibit myeloid differentiation hinges on its ability to bind COP1, formin a complex that causes targets the myeloid differentiation factor C/EBP? for degradation. Here, we propose a model in which TRIB directs the specificity of COP1 for C/EBP? by binding directly to one domain of COP1. If correct, this model would invite the design of inhibitors that target the TRIB-COP1 interface. The work proposed here will be conducted under the mentorship of Dr. Stephen Blacklow. The candidate is a physician-scientist with board certification in Clinical Pathology and subspecialty training in Transfusion Medicine. The studies outlined here will form the foundation for the candidate's independent research. Aim 1. To determine the structural basis for assembly of functional TRIB-COP1 complexes. Subaim1.1 To determine the mode of binding between Tribbles and COP1. Our first structural target will be a complex between the TRIB-binding region of COP1 bound to the COP1 binding motif of TRIB1, which will give new insights into TRIB structure-function relationships and provide the foundation necessary to design selective inhibitors that directly disrupt the TRIB-COP1 interaction. Subaim1.2 To identify the mechanism by which Tribbles and COP1 recognize substrates destined for degradation. We will combine mutagenesis with biophysical and structural approaches to determine the basis for substrate recognition, with the long-term goal of solving the structure of a ternary complex containing a TRIB protein, COP1, and substrate. Aim 2. To discover new, functionally important substrates of TRIB-COP1 complexes in mammalian cells. We are now discovering new targets of TRIB-COP1 mediated degradation using a substrate-trapping strategy in which the COP1 ligase activity has been inactivated. Completion of this aim will deepen our understanding of TRIB function in mammals, and provide additional insight into the range of substrates targeted for degradation by TRIB-COP1 complexes in mammalian cells. Together, completion of these aims will reveal the molecular mechanism by which TRIB-COP1 complexes promote AML, and serve as the template for future design of targeted inhibitors of these complexes.